1. Field of Invention
The invention relates to a process and a device for detecting electrically conductive particles in a liquid flowing through a pipe by means of eddy currents.
2. Description of Related Art
German Patent Application DE 2 108 717 A1 describes a process and device of the type to which the present invention is directed in which two induction coils are located in two branches of an alternating current bridge circuit, the other two branches of which are formed by the halves of another coil. Liquid flows through the coils in the axial direction and the coils can be arranged in succession in the flow direction, the impedance changes which are caused by the passage of the particles and the difference of the impedance change in both coils being evaluated. An arrangement is shown in which the liquid flow is divided into two parallel component sections which each flow through one of the two coils, in which case an axial offset of the coils not necessary.
A similar device is described in German Patent Application DE 28 40 358 A1.
The company momac GmbH & Co. KG, 47408 Moers, Germany sells a device under the name “metalscan” in which three coils are arranged in succession in the flow direction, the first and the last coil acting as the transmitter coils and the middle coil acting as the receiver coil to detect passage of electrically conductive particles from a lubricant circuit. The first and the last coil are polarized in reverse.
Other devices in which the signal from the induction coils through which a liquid has flowed is used for particle detection are described for example, in International Patent Application Publications WO 2004/081608 and WO 2004/104561, European Patent Application EP 0 778 937 A2 (which corresponds to U.S. Pat. No. 5,811,664) and European Patent Application EP 0 451 209 B1.
German Patent Application DE 39 31 497 A1 discloses a process for inductive detection of particles in lubricants, a coupling coil embedded in a coil through which flow takes place axially being resonantly excited and the passage of particles being detected using the energy removed from the coil system by the eddy currents. In this connection, the particle size is determined from the signal amplitude. In order to prevent adulteration of the measurement by the coil sensitivity which decreases in the coil middle as compared to the coil edge, a swirl generator in the passage provides for the particles passing the coil to always be near the coil wall.
German Patent Application DE 31 17 319 A1 and corresponding U.S. Pat. No. 4,380,924 describe detection of the flow velocity of a liquid metal by means of eddy current measurement using a cross correlation function.
German Patent Application DE 40 14 756 A1 describes determination of the velocity of a body or material by means of eddy current measurement, a correlation function being formed.
U.S. Pat. No. 3,575,050 and German Patent Application DE 28 50 246 A1 mention that there are flowmeters based on eddy currents.
Furthermore, it is known that, in eddy current testing of metallic workpieces, a coil arrangement can be used in which there are subtractively connected receiver coils which are spaced apart in the lengthwise direction of the workpiece and which are surrounded externally by a transmitter coil which is located coaxially thereto. In eddy current testing, then, the workpiece is pushed through the interior of the two receiver coils. The transmitter coil forms the primary side and the receiver coils form the secondary side of a transformer arrangement. One example of this arrangement can be found in European Patent Application EP 1 189 058 A2 corresponding U.S. Pat. No. 6,566,871.
In eddy current testing of workpieces, the fact is used that defects in the material of the workpiece hinder the propagation of eddy currents which are induced by means of the transmitter coil; this acts on the electromagnetic field which has been produced by the eddy currents and which is detected in turn by a sensor which can be the transmitter coil itself or at least one separate receiver coil. If only a single separate receiver coil is provided, this arrangement is called an “absolute coil.” Two or more measurement coils can be subtractively connected; this is then called a “difference coil” and enables, for example, temperature drift to be neutralized. If more than two receiver coils are used, this arrangement is also called a “multi-difference coil”.
Similarly, electrically conductive particles in a liquid which is flowing through the coils cause eddy current losses which, in turn, are reflected in a measurable impedance change of the coils. In this way, by means of an inductive coil arrangement, electrically conductive particles in a liquid flowing in a tube can be detected. This is especially advantageous when the concentration of metallic particles in the lubricant circuit of a machine is to be detected in order to draw conclusions about the machine state (the concentration of metallic particles is generally a measurement for machine wear).